A ceramic body with a coaxial hole bored through its length forms a resonator that resonates at a specific frequency determined by the length of the hole and the effective dielectric constant of the ceramic material. The holes are typically circular, or elliptical. Combining multiple resonators may form a dielectric ceramic filter. The holes in a filter must pass through the entire block, from the top surface to the bottom surface. This means that the depth of a hole is the exact same length as the axial length of a filter. The axial length of a filter is selected based on the desired frequency and specified dielectric constant of ceramic.
The ceramic block functions as a filter because the resonators are inductively coupled and/or capacitively coupled between every two adjacent resonators. These couplings are formed by the electrode pattern designed on the top surface of the ceramic block, plated with a conductive material such as silver or copper. More specifically and with reference to FIGS. 10A-D, a ceramic block 101 is shown with two holes 103 and 105. All surfaces, except for the front open face 107 through which the two holes 103 and 105 extend, are plated with silver. Due to the size of the holes, their proximity and the conductive coating, the two holes 103 and 105 are inductively and capacitively coupled to each other. However, block 101 will not perform as a filter because these couplings cancel each other out.
To form a filter, a pattern of conductive material is printed on face 107, as shown in FIG. 10B. In this embodiment the patterns A and A enhance the capacitive coupling between holes 103 and 105. While the capacitive coupling is enhanced, the inductive coupling remains substantially unaffected. This is because inductive coupling is mostly a function of the hole diameter, shape and spacing between holes. These parameters are the same in FIGS. 10A and 10B.
The capacitive coupling can be regulated in FIG. 10B by adjusting parameters L and G. By decreasing G or increasing L, the capacitive coupling is strengthened. The capacitive coupling can also be weakened such that the inductive coupling is stronger, by printing line M on open face 107. The simple line M in FIG. 10C has a greater diminishing effect on the capacitive coupling of the block filter 101, than the broken line M of FIG. 10D.
Ceramic filters are well known in the art and are generally described for example in U.S. Pat. Nos. 4,431,977; 4,716,391; 4,954,796 and 5,783,980, all of which are hereby incorporated by reference as if fully set forth herein.
With respect to its performance, it is known in the art that the band pass characteristics of a dielectric ceramic filter are sharpened as the number of holes bored in the ceramic block are increased. The number of holes required depends on the desirable attenuation properties of the filter. Typically a simplex filter requires at least two holes and a duplexer needs more than three holes. This is illustrated in FIG. 1 where graph 10 represents the filter response with fewer holes than graphs 12 and 14. It is apparent that graph 14 which is the response of the filter with the most holes, is the sharpest of the three responses shown. Referring to FIG. 2, it can be seen that the band pass characteristic of a particular dielectric ceramic filter is also sharpened with the use of trap holes bored through the ceramic block. Solid line graph 21 represents the response of a filter without a high end trap. Dashed line graph 23 represents the response of the same filter with a high-end trap.
Trap holes, or traps as they are commonly referred to are resonators which resonate at a frequency different from the primary filter holes, commonly referred to simply as holes. They are designed to resonate at the undesirable frequencies. Thus, the holes transmit an input signal at the desirable frequencies while the traps remove the input signal at the undesirable frequencies, whether low end or high end. In this manner the characteristic of the filter is defined, i.e. high pass, low pass, or band pass. The traps are spaced from holes a distance greater than the spacing between holes so as to avoid mutual interference between the holes and traps. As shown in FIG. 3, whereas holes 31 are separated from each other a distance D, a distance of 2 D is placed between trap 33 and the hole nearest to trap 33. The precise distance D is one of design choice for achieving a specified performance. D typically falls within a preferred range of 1 to 10 mm. Traditionally, the traps will be spaced from 1.5 D to 2 D from the holes.
Conventionally the holes 41 and traps 43 in a ceramic filter are positioned along a straight line, as shown in FIG. 4. This design together with the spacing requirements addressed above limits the extent to which a filter may be reduced in size. Specifically, the performance characteristics of a given filter are a function of its width, length, number of holes and diameter of holes. The usual length is 2 to 20 mm. The width of a filter is a function of the number of holes in the filter. Typically, the width of the block filter ranges from 2 to 70 mm. Reducing the number of holes, the diameter of the holes, or the spacing between holes, will effect the performance of the filter. Accordingly, it is desirable to design a dielectric ceramic filter which can effectively reduce the size of a given filter while maintaining its given performance characteristics.